water heater for new tub and shower

If

the existing house works (let's assume that it does and you have never run out of hot water) I would take that at face value, meaning you meet the demand but probably do not have much to spare. Properly done you should assess the fixture unit count of your house as it is, use diversity (Hunter's Curve). You will probably still find out that while you may have some surplus you probably do not have enough for a high-volume user like a hot tub.

Add to that multiple heads of a spa shower (6 heads at 2.5 gpm =15 GPM. Off a 50-gallon tank that is a 3+ minute shower. Is that even enough time to play "drop the soap"? )


I would recommend piping in parallel not in series. Piped in series just takes the load off of the second heater, allows boosting of temperature and unless you put in a charging or recirculation loop, it will not add flow capacity. The common series pipe can only carry so much. Run two in parallel and your potential doubles.

The question also becomes, what does the hot tub require? Not just the volume but the ability to heat it in a given amount of time? Will you drain it each time or heat it indirectly like a swimming pool? If the volume of the HT is 50 gallons do you have enough time to fill it before it cools? Pressure and piping sized adequately?

Generally I concur with your plumber's recommendation that you add capacity but would also like to see them manifolded together (in parallel as stated) for back up one to the other and to absorb any surplus your existing heater now has. You may want to add storage capacity if the use is sporadic (large volume of heated water that can recover temperature over time).

If you have a HW boiler, an indirect heater using all of the boiler capacity for a quick charge is another good idea. Especially if you do not use your tub in the coldest weather and have the capacity available. Just another thought.